Here’s Exactly How Exercise—Or Lack of It—Affects Aging Muscles. This Just Might Change Your Life

Under the glare of overhead lights, the paper-thin, ruby-red shavings of Rosangela Santiago’s muscle glow like fine gems. I can’t believe those just came out of my thigh, she thinks, watching as a scrubs-clad researcher places the five tiny slices into individual vials. Normally on a Tuesday morning, Santiago would be sitting at the office, processing invoices and getting a slow start to her day. But today she’s lying on a surgical table at the Translational Research Institute for Metabolism and Diabetes in Orlando, in the midst of a muscle biopsy and officially now a lab rat in the race for a more sophisticated understanding of why we get fatter, weaker, and sicker with age—and how it’s all intricately linked to the meaty fiber hidden beneath our skin.

A few moments from now, the research team will carefully preserve Santiago’s tissue in a liquid nitrogen tank chilled to minus 328 degrees, adding it to the thousands of other samples from people who, like Santiago, have volunteered for the TRI’s studies—studies that are contributing to a dramatic shift in how we think about muscle. For within those vibrant slivers of Santiago’s quadriceps lies precious information about her current health, as well as a forecast of what her future may hold. New science shows that it’s not just how much muscle you have that’s important—it’s what’s inside it that matters most. Metabolically active muscle—the kind that’s devoid of harmful fat and teeming with mitochondria, the metabolism-boosting powerhouses within cells—has a profound influence on everything from your weight to your energy levels to your risks of diabetes and heart disease. Even your chances of surviving a hospital stay or beating cancer are affected by the health of your muscle. This fitter, leaner, more-bang-for-your-buck tissue is so vital, in fact, that pharmaceutical companies are in hot pursuit of drugs that will enhance the metabolic capacity of muscle and help us maintain it as we age, as well as during hospital stays or times of illness.

Like most of us, Santiago doesn’t yet know that diminishing muscle quality is a large part of the reason she’s been so tired lately and why those few extra pounds have started clinging to her like swirls of frosting to a cake. She’s volunteered in hopes that the study will help her better understand how her metabolism works and inspire her to start—and stick with—a workout program. For the past decade, as the demands of work, school, and raising three sons have piled up, she’s done little more than trek from her house to her car. Ten thousand steps a day? She’s lucky if she takes 500. By the lab’s calculations, she falls solidly within the range of “total couch potato” and is inching closer to becoming diabetic. It’s the wake-up call she needs, but she can’t help but feel scared: How did I let myself get so far off track?

Aside from the fatigue, however, Santiago, 38, feels relatively healthy, and therein lies the big danger for her and the rest of us: Losing metabolically charged muscle is a sneaky, below-the-radar problem that starts in our 30s. Without your knowing it, as the years go by and you spend more time sitting and less time moving, your muscle loses its zing. Mitochondria slowly decline in number and vigor, while pernicious fat starts to seep in, marbling your muscle until it looks more like rib eye than lean top sirloin. Little by little, your health and vitality decline, until one day you realize that your favorite clothes no longer fit, you’re exhausted by a trip to the grocery store, and your doctor is writing you a script for metformin to control your diabetes.

Unless, of course, you know how to turn this destructive situation around.

After hearing Santiago’s story, I’m halfway through my own tour of the TRI’s high-tech muscle lab when I start to wonder what the inside of my own muscle looks like. “So how do I know if my mitochondria levels are low or if my hamstrings are getting fat?” I ask Bret Goodpaster, one of the TRI’s fit, brainy researchers. He’s promised to help me understand how Santiago and the rest of us can naturally harness the anti-aging, disease-preventing power that lies buried in our quads and glutes.

He waves me over to a tidy row of microscopes that he and his colleagues use to peer inside muscle’s mysterious inner world. As Santiago did, I’d need to undergo a biopsy and have the TRI slide a slice of tissue under one of these high-powered lenses to determine my mitochondria levels, as well as get an MRI scan to check for fat infiltration—procedures that are normally done only in a research environment. Or, he tells me, anyone can make an educated guess about the state of their tissue. The single most important factor: how often you put one foot in front of the other.

It turns out that, like so much having to do with our health, the wealth of our mitochondria and the leanness of our muscle is inexorably bound to how much—or how little—we move. In terms of mitochondria, it’s a classic case of supply and demand. Whether you’re doing a quick walk or a hard workout, when muscles need energy, they call on these microscopic generators to turn glucose and fat into adenosine triphosphate, the cellular fuel your muscles use to fire. Stay away from the gym too long and your hyperefficient body notices that it doesn’t need as many of these teeny power plants, so it tells the extras to take a hike—and the ones that do stick around become sluggish and less efficient. Over time, your internal power source dwindles from a roaring fire to a flickering candle, sapping both your energy and your capacity to burn fat.

The thought of becoming inept at torching belly flab has me worried. I raise my concerns to Goodpaster, who quickly points out that the ramifications go far beyond how I fit into my jeans. Vibrant, plentiful mitochondria also help keep muscle tissue lean, so the fewer of the mighty mitos you have, the easier it is for fat to worm its way deep inside. And fat can be toxic in muscles, because it can cause the tissue to become less responsive to insulin, making it easier to gain weight and harder to lose it—and putting you smack in the crosshairs of diabetes.

If that’s not alarming enough, research shows that fatty muscle is more likely to cause weakness and loss of mobility—making you doddering before your time—than dwindling muscle tissue itself. It doesn’t take a PhD to do the math: When you are weaker and tired and have less stamina, exercise can go quickly from challenging to painful to less appealing than filing taxes. Get too comfortable on the couch and you’ll perpetuate the cycle, losing more mitos and stock­piling more fat with every sofa-bound day that goes by.

Ian Allen and Bob Croslin

My mind wanders again to the small layer of blubber that, now that I’m 52, has settled around my midsection. Even though I hike and run 6 days a week, I haven’t always been so virtuous. When my kids were younger, there were months-long stretches when I was way too busy-tired-overwhelmed to do anything more strenuous than shuffle through the grocery store. I can’t help but wonder: Could that same stealthy stuff have invaded my muscle? “So how far off is that miracle drug the pharmaceutical companies are working on?” I ask Goodpaster jokingly.

Years, if ever, he tells me. Then he lights up like a kid visiting nearby Disney World. But that’s the cool part, he says. If you keep exercising, you may never need it.

Exactly how exercise unlocks muscle’s anti-aging potential is still a bit of a mystery, but it appears that aerobic workouts—whether you’re running on the treadmill or taking a spin on your bike—spark the cellular system that creates new mitochondria. As your muscles demand energy, the mitos themselves, along with enzymes in the tissue, switch on genes that start transcribing mitochondrial DNA. The more you stick with it, the more you make, and the more efficient your muscles become at burning fat and providing energy. It doesn’t take much to make an impact: Research shows that after as few as 3 to 7 days of brisk walking for 45 minutes, you’ll start to spur the growth of new mitochondria.

“What if you’ve been sedentary for most of your adult life?” I ask, thinking of older people I’ve known who’ve slowly become shadows of their former vibrant selves. “Is it really possible to get your muscle’s mojo back?”

Although some people have a more robust response than others—an unfair advantage that can probably be traced, along with other enviable traits, to good genes—anyone, no matter their age, can create more youthful muscle, says Goodpaster, who’s seen dozens of turn-back-the-clock transformations in his 20 years as a muscle evangelist. In one study of sedentary men and women in their late 60s, participants who walked on a treadmill or rode an exercise bike for 30 to 40 minutes 4 to 6 days a week increased their mitochondria volume by as much as 68 percent in just 12 weeks. In another study, researchers found that inactive people in their 70s and 80s larded their muscle with 18 percent more fat in a single year—a level that can lead to a life-changing drop in strength and mobility—while active people the same age didn’t gain an ounce.

The numbers can change drastically, almost shockingly, when exercise is added. Goodpaster refers me to Colette Satler, a 65-year-old MRI technologist in Pittsburgh who participated in one of his muscle studies in 2009. At the start, she had recently lost a significant amount of weight through bariatric surgery but still felt out of shape. She was even thinking of retiring early because climbing the stairs at the hospital where she worked left her winded. Within 6 months of starting a regular exercise program (she pedaled a stationary bike or walked on a treadmill for 60 minutes 3 or 4 days a week), not only had her mitochondria volume increased by 30 percent, but she also slashed the fat in her muscle by nearly half. Instead of retiring, she completed an item on her bucket list: a 300-mile bike ride from DC to Pittsburgh.

More curious than ever about the content of my own muscle, I ask Goodpaster if he’ll biopsy mine. My 81-year-old mom walked regularly at my age; now, sidelined for a decade by a degenerative spine condition, she can barely make it 2 blocks. Does the same fate await me? Have I been doing enough? He declines, reminding me that I have to be part of a study to undergo the procedure. A few weeks later, I shell out for an MRI, so I can at least find out if all my sweating is keeping fat from colonizing my lean tissue.

As I’m lying inside the coffinlike tube of the scanner, the technician aims the magnetic pulses at my left calf, a muscle I’ve toned through years of running up hills. It’s hard not to fidget when my mind is swimming with visions of gooey, marbled tissue. But soon I’m heading home, dozens of shadowy images in my grasp.

I pore over them, trying to understand what I’m looking at. At a loss, I ship them off to Goodpaster. Within 24 hours, I get just what I’m looking for in midlife: reassurance. According to him, my muscle tissue is as healthy as a 25-year-old’s. With my own anxiety allayed, I check back in with Santiago to see how she’s doing.

For the past 6 weeks, she’s been making a crack-of-dawn trek to the TRI to ride a stationary bike and walk on a treadmill for up to 90 minutes. Instead of dragging through the beginning of her day, she feels energetic and alert, and by 6 pm, when she’s sitting in her night class, she has energy to spare. According to the researchers, she’s also put on a full pound of healthy, fit muscle. But when she feels this good, she doesn’t need a lab result to tell her the most important thing: “Exercise makes me feel happier, leaner, and younger,” she says. “It’s the best drug out there.”

A Part of Hearst Digital Media
Runner's World participates in various affiliate marketing programs, which means we may get paid commissions on editorially chosen products purchased through our links to retailer sites.